1. Field of the Invention
The invention relates to elastomeric tubes, more specifically, to tubes having at least two silicone rubber compositions blended at an interface to form a gradient. In particular, the invention relates to medical-grade silicone rubber tubes useful as cardiac pacemaker lead insulation.
2. Description of the Prior Art
Elastomeric tubes are used in a variety of applications and produced in numerous ways. One type of elastomeric tubes, silicone rubber tubes or lumens, are widely used in medical applications. For example, silicone rubber lumens are often used to insulate the electronic lead portion of implantable cardiac pacemakers. This is largely due to their unique combination of properties, including low toxicity, high thermal stability, good moisture resistance, excellent flexibility, high ionic purity, low alpha particle emissions, and good adhesion to various substrates.
To form silicones, one can begin with organopolysiloxane compositions which are then cured. The curing method can be, for example, through hydrosilylation where an addition reaction of silicon-bonded lower alkenyl radicals with silicon-bonded hydrogen atoms occurs. Further, curing can be catalysed with, for example, platinum. Another curing option is peroxide initiated curing, where peroxide free radicals initiate the combination of pendant vinyl groups to form cross links.
Methods and materials for preparing linear, branched, and cyclic organopolysiloxanes and other starting materials are well known in the art, as are various curing methods. The goal of the curing is to form some sort of cross-link or junction between and among the polymer strands to create a continuous network or gel.
Presently, silicone rubber lumens are selected for use based on a particular desired property. One type of silicone may be useful where tear resistance is desired; another useful where crush resistance is necessary. In some cases, dual- or multi-layer silicone rubber lumens have been produced in an attempt to maximize desirable properties and/or to provide differing properties at different surfaces of the lumen.
There are inherent drawbacks with present silicone rubber lumens. Single-layer lumens may not possess all desired properties, whereas dual-layer lumens often have problems with delamination at the interface of the two silicones, particularly when subjected to shear forces. Further, as emerging applications for silicone lumens grow, the specific demands of new products require new and improved lumens. There is a need to meet these challenges.